Method of making ceramic lid assembly for hermetic sealing of a semiconductor chip

ABSTRACT

A ceramic lid assembly includes an integral heat fusible layer defining a hermetic sealing area provided around the periphery of a ceramic lid for hermetic sealing semiconductor chips in a flat pack. The integral heat fusible layer includes a metallized layer, an oxidation inhibiting layer, and a preflowed solderable layer in registration with each other in the hermetic sealing area.

This is a division of application Ser. No. 122,527 filed Feb. 19, 1980,U.S. Pat. No. 4,291,815.

BACKGROUND OF THE INVENTION

This invention relates in general to hermetic sealing of semiconductorchips, and more particularly, to a ceramic lid assembly having aselectively deposited integral heat fusible layer for hermetic sealingof a package containing a semiconductor chip.

Integrated circuits in the form of one or more semiconductor chips areassembled for utilization in small packages generally known as flatpacks. The flat pack encloses the semiconductor chip for protection ofthe various components therein to prevent damage from handling as wellas atmospheric and other destructive conditions. The flat pack is formedof a package having a central opening on one major surface. Thesemiconductor chip is placed within a cavity of the flat pack defined bythe central opening and is connected electrically by leads extendingthrough the walls of the package. It has become conventional to providea hermetically sealed cover over the central opening to protect thesemiconductor chip.

Heretofore, it has been the practice to hermetically seal thesemiconductor chip within the cavity of the flat pack by placing apreformed ring of heat fusible material, such as a gold-tin eutecticsolder, on the sealing area of the package surrounding the cavity, andoverlying the central opening with a gold plated Kovar lid. Upon heatingthe thus formed assembly, the gold-tin eutectic solder forms a hermeticseal between the package and the gold plated lid.

The solder materials of the preforms are generally brittle and theirdimensions are so small that the preforms are very fragile and extremelydifficult to handle during assembly. Because of the difficulty ofhandling such preforms, it has also been difficult to insure accurateregistration between the preform, the peripheral region of the lid andthe sealing area surrounding the cavity of the package. As aconsequence, there has been a substantial yield loss in the assembledflat packs due to defects in the hermetic seal. In addition, with theprice of gold increasing to unforeseen limits, there is a real economicneed to minimize or completely eliminate the use of gold in providing ahermetically sealed flat pack.

One hermetic sealing technique which partially overcomes the above noteddisadvantages is described in U.S. Pat. No. 3,874,549. In general, agold plated Kovar lid for hermetic sealing of the package issuperimposed with a heat fusible preform. The precise registrationbetween the preform and the lid is achieved by spot welding the preformat a plurality of space locations about the peripheral area of the lid.

However, the welding of the preform to the lid greatly increases therisk of trapping dirt and dust therebetween which ultimately causesleaks in the hermetic seal. Visual inspection of the welded preform tothe lid will not give any indication of the subsequent quality of thehermetic seal. Accordingly, defects in the hermetic seal are not noticeduntil the flat pack assembly is fully fabricated, resulting in anadditional loss in yield. As these lids with welded preforms arecommingled in automatic processing equipment, there is a tendency forthe corners of the preforms to lift up and break off, further reducingthe overall yield and quality of the hermetic seal.

With the semiconductor chip mounted in the cavity of the package,electrical connections must be made from the terminal pads on thesemiconductor chip to the leads extending through the package walls. Themost widely used method is thermo-compression bonding. Gold wire aboutone mil in diameter is used in a process called Ball Bonding. The goldwire, if not precisely aligned within the cavity of the flat pack, is indanger of making contact with the conductive gold plating on the Kovarlid. In the event of contact by the gold wires with the conductive goldplated lid, the semiconductor chip becomes shorted and inoperative. Theuse of a dielectric lid results in difficulty in obtaining a hermeticseal as it is known that the gold-tin eutectic solder used to fabricatethe preforms will not wet and form a hermetic seal to dielectricmaterials such as ceramics.

Thus, there is heretofore an unsolved need to provide a dielectric lidfor use in hermetically sealing semiconductor devices which overcomesthe above-noted disadvantages resulting in the use of known lids andhermetic sealing techniques.

SUMMARY OF THE INVENTION

It is broadly an object of this invention to provide a lid assembly forhermetic sealing of a semiconductor chip which overcomes or avoids oneor more of the foregoing disadvantages resulting from use ofconventional lids. Specifically, it is within the contemplation of thepresent invention to provide a dielectric lid with an integral heatfusible layer disposed around the peripheral region of the lid inregistration with a sealing area.

It is a further object of the present invention to provide a dielectriclid assembly for hermetic sealing of a semiconductor chip whicheliminates or greatly reduces the utilization of gold as a materialcomponent.

It is still a further object of the present invention to provide adielectric lid assembly having an integral heat fusible layer thateliminates the separate handling of fragile preforms used in hermeticsealing of semiconductor chips.

It is still a further object of the present invention to provide adielectric lid assembly having an integral heat fusible layer applied bytechniques that improve the registration of the heat fusible layer withthe sealing area of the dielectric lid.

It is still a further object of the present invention to provide adielectric lid assembly having an integral heat fusible layer which willmaintain registration with the sealing area formed around the centralopening of a flat pack during the hermetic sealing of a semiconductorchip.

It is still a further object of the present invention to provide adielectric lid assembly having an integral heat fusible layer whoseuniformity is an indication of the quality to be obtained in thehermetic seal.

It is still a further object of the present invention to provide adielectric lid assembly of ceramic material which eliminates thepossibility of electrical shorts between the gold ball bonded wires.

In accordance with the present invention there is provided a lidassembly for hermetic sealing of a package containing a semiconductorchip including a lid of dielectric material and an integral heat fusiblelayer deposited on the lid in a peripheral region defining a hermeticsealing area.

BRIEF DESCRIPTION OF THE DRAWINGS

The above brief description, as well as further objects, features andadvantages of the present invention will be more fully understood byreferences to the following detailed description of a presentlypreferred, nonetheless illustrative, ceramic lid assembly in accordancewith the present invention when taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a prospective view of a ceramic lid assembly having anintegral heat fusible layer for hermetic sealing of a semiconductor chipwithin a flat pack;

FIG. 2 is a prospective view of a ceramic lid assembly in registrationwith a semiconductive chip for hermetic sealing; and

FIG. 3 is a melting profile for a gold-tin alloy preform in a three-zonefurnace.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to the drawings, there is shown generally in FIG.1 a lid assembly 100 including lid 102. The lid 102 has at least oneplanar surface 104. Aligned around the periphery of the surface 104 is ametallized heat fusible layer 106. The metallized layer 106 is providedin the form of a ring. The term "ring" is used herein in its genericsense to include a closed loop of any configuration corresponding to theperipheral area of the lid 102, usually round or rectangular.Optionally, over the metallized layer 106 and in registration therewithis an oxidation inhibiting layer 108. Deposited over the oxidationinhibiting layer 108 is a solderable heat fusible layer 110 inregistration with the metallized layer 106. The metallized andsolderable layers 106, 110 and optional oxidation inhibiting layer 108provide an integral heat fusible layer 112 around the periphery of lid102 to define a hermetic sealing area 114 in the form of a ring having acentral opening 116.

The lid 102 is of substantially non-conductive or dielectric materialhaving a thickness range of from 0.010-0.040 inch. Ceramic material isgenerally preferred as it is inexpensive, easily metallized and has acofficient of thermal expansion which matches that of the semiconductorflat pack. Ceramic material found suitable includes the oxides ofaluminum, beryllium and magnesium. For example, polycrystalline aluminumoxide commercially available from Coors Porcelin Co., Colorado,designation ADS-96F and 3M Technical Ceramic Product Div., Tenessee,designation Alsimag 614 are excellent ceramic material. Single crystalsapphire is ideally suited for use as a lid in hermetic sealing ofEPROMs by being transparent to ultra-violet light. The use of singlecrystal sapphire provides a large window opening for easy accessibilityto the semiconductor chip for erasing and programming the Read OnlyMemory.

In general, lids made of ceramic material will not be wetted by mostcommercially available alloys used for the solderable heat fusible layer110. Failure of the solderable layer 110 to adhere to the ceramic lid102 will prevent the formation of quality hermetic seals. The solderablelayer 110 is assured adherence to the ceramic lid 102 by depositing anadherent wettable metal layer around the periphery of the ceramic lid102 such as metallized layer 106.

There are commercially available screen printable compositions in theform of metallized pastes which will effectively metallize the surface104 of ceramic lid 102. These metallized pastes are mixtures of metalpowders suspended in organic additives to impart fluid properties to thepaste in providing for screen printing application. Known organicadditives include methyl, ethyl, butyl and propyl alcohol. Themetallized pastes can also include a permanent binder of low meltingpoint glass frit which bonds the resulting metallized layer to thesubstrate. These glass frits include bismuth oxide, cadmium oxide andlead borosilicate. The metallized pastes are screen printed to asubstrate and subsequently baked at a low temperature, such as 125° C.to burn out the volatile organic additives. The dried metallized pasteis fired at an elevated temperature, such as 750°-1050° C. to fuse themetal powders and bond the resulting metallized layer to the substrate.A preferred composition for metallized layer 106 is a mixture ofmolybdenum/manganese, 80%/20% by weight respectively, commerciallyavailable in a screen printable paste from Electro-Ceramic, New Jersey,for firing between 1300°-1600° F. In addition, palladium, silver andgold screen printable pastes that include a glass frit binder arecommercially available from Alpha Metals, New Jersey and E. I. duPont,Delaware, for firing between 1400° F.-1850° F. can also be used.

The hermetic sealing of the flat pack is provided by solderable layer110 which can be in the form of a screen printable metallized paste aspreviously described, or as a stamped preform of a soft solder alloy.Stamped preforms are commercially available from Consolidated RefiningCompany, New York, in a wide range of alloys having melting pointsgenerally below 450° centigrade. The stamped preforms are fabricatedfrom alloys of lead, tin, bismuth, gold, silver, antimony and zinc. Thepreferred composition of solderable layer 110 is an alloy oflead/tin/silver, 88%/10%/2% by weight, respectively, and gold/tin,80%20% by weight, respectively. Selected compositions of the stampedpreforms in the form of a screen printable metallized paste arecommercially available from Alpha Metals and E. I. duPont.

The adhesion of the solderable layer 110 to the metallized layer 106 canbe improved by the inclusion of an oxidation inhibiting layer 108. Theoxidation inhibiting layer 108 enhances the solderability of themetallized layer 106 by the solderable layer 110 by prevently surfaceoxidation of metallized layer 106 which could occur during theconstruction of the lid assembly 100. The oxidation inhibiting layer 108is a thin layer of from 50-100 micro inches of nickel, gold or copperwhich may be deposited by electroplating. In the preferred embodiment,nickel has been found to be of the greatest effectiveness in inhibitingoxide formation at a low material cost.

In one embodiment, an aluminum oxide polycrystalline ceramic lid 102,designation ADS-96F or Alsimag 614, is provided. A one mil thick layerof molybdenum/manganese paste is silk screened around the peripheralarea of a cleaned surface 104 of lid 102. The silk screening operationis carried out according to well known techniques such as those taughtin "Thick Film Hybrid Microcircuit Technology," Donald W. Hamer andJames V. Biggers, Wiley-Interscience, a Division of John Wiley & Sons,Inc., New York, Copyright 1972; and "Thick Films Technology and ChipJoining," Lewis F. Miller, Gordon and Breach Science Publishers, Inc.,New York, Copyright 1972.

The molybdenum/manganese layer is in the shape of a ring having a widthof approximately 0.1 inches and provides the metallized layer 106. Themetallized layer 106 is first dried at low temperature to remove thevolatile organic additives and then fired in an inert or reducingatmosphere at 1400° centigrade to fuse the molybdenum/manganese pasteinto the metallized layer 106. Inert atmospheres of argon, nitrogen andhelium, and reducing atmosphere of wet hydrogen/nitrogen, 35%/65% byvolume, respectively, are useful to prevent the oxidation of the surfaceof the metallized molybdenum/manganese layer.

To enhance the solderability of the metallized molybdenum/manganeselayer and to prevent subsequent surface oxidation, a 50-100 micro inchlayer of nickel is electroplated onto the surface of the metallizedmolybdenum/manganese layer.

A stamped preform about 2 mils thick in a ring shape of a solderablealloy of gold/tin, 80%/20% by weight, respectively, having a meltingpoint of about 280° centigrade is placed within the cavity of a carbonboat constructed to receive the stamped preform and ceramic lid 102. Thestamped preform provides the solderable layer 110. The ceramic lid 102is placed within the carbon boat with the metallizedmolybdenum/manganese layer facing downward so as to be in registrationwith the stamped preform. A forging weight is placed on top of ceramiclid 102 to enhance the wetting of the stamped preform to thenickel-plated metallized molybdenum/manganese layer as the stamp preformbegins to melt during a preflowing operation.

The carbon boat is transported through a multi-zoned furnace at a speedof 10 inches per minute. The furnace is constructed of three zoneshaving independent temperature control. The first zone is set at 250°centigrade, the second zone at 240° centigrade, and the third zone at505° centigrade. Optionally, a fourth zone may be used as a coolingchamber to cool the ceramic lid assembly 100 before exiting from thefurnace. The actual temperature attained by the ceramic lid assembly 100passing through the furnace is shown in the melting profile of FIG. 3. Adisassociated ammonia atmosphere at a flow rate of approximately 20 SCFHis introduced into the furnace as a reducing atmosphere to prevent theformation of oxides.

The stamp preform preflows onto the nickel and metallizedmolybdenum/manganese layer to form an integral heat fusible layer 112 inthe hermetic sealing area 114 of the ceramic lid 102.

In another embodiment, the stamped preform is a solderable alloy oflead/tin/silver, 88%/10%/2% by weight, respectively, having a meltingpoint of about 268°-299° centigrade. The ceramic lid assembly 100 isplaced within a carbon boat and passed through the multi-zone furnace atabout 5 inches/minute belt speed to melt the preform and produce theintegral heat fusible layer 112. The first zone is set at about 380°centigrade, the second zone at about 400° centigrade, and the third zoneat about 200° centigrade.

In another embodiment, the solderable layer 110 is provided as a 2 milthick screen printable metallized paste of 88% lead, 10% tin and 2%silver. The solderable layer 110 is screen printed over the nickeloxidation inhibiting layer 108 in registration with the metallized layer106 of metallized molybdenum/manganese. The ceramic lid assembly 100 isplaced in the carbon boat to fire and preflow at about 800° centigradethe solderable metallized paste to the nickel and metallizedmolybdenum/manganese layer.

As thus described, the ceramic lid assembly 100 is provided with ametallized layer 106 protected from oxidation by an oxidation inhibitinglayer 108 including a solderable layer 110 in registration with eachother around the periphery of surface 104 of the ceramic lid 102. Uponfiring of the ceramic lid assembly 100 a preflowed integral heat fusiblelayer 112 provides a hermetic sealing area 114. The ceramic lid assembly100 eliminates the handling of fragile stamped preforms, provides foraccurate registration of the integral heat fusible layer 112 with theceramic lid 102 and the flat pack, eliminates the use of expensive goldas a material component, and provides for the visual inspection of theintegral heat fusible layer to determine the quality of the subsequenthermetic seal.

Hermetically sealed flat packs according to the present invention havebeen found to surpass leak testing according to the procedures of MilSpec. 883B. The hermetically sealed flat packs are placed in a heliumatmosphere at 35-50 p.s.i. for two hours at room temperature. Using ahelium leak detector, leak rates of less than 1×10⁻⁸ were observed.Additional leak testing by placing the flat pack in cold freon,temperature cycling from -65° centigrade to +150° centigrade, placingthe flat packs in a 24 hour bake at 150° centigrade, and storage testingfor 168 hours all resulted in acceptable leak rates for flat packshermetically sealed using ceramic lid assemblies according to thepresent invention.

In FIG. 2, the method of hermetic sealing of the ceramic lid assembly100, fabricated accordingly to the present invention, to the flat pack118 having a cavity 120 in which is disposed a semiconductor chip 122,is shown. A flat pack 118 may be either of ceramic or metallic material;if ceramic, a conductive ring 124 is fused to the flat pack surroundingthe cavity 120. The flat pack 118 and the ceramic lid assembly 100 inregistration therewith, is passed through a suitable belt furnace forfusing the integral heat fusible layer 112 with ring 124 to hermeticallyseal the semiconductor chip 122 in the cavity 120.

Accordingly, by using the method and ceramic lid assembly according tothe present invention for hermetic sealing of a semiconductor chip, thehandling of fragile stamped preforms is avoided. Further, the methodinsures that the integral heat fusible layer 112 remains in registrationwith the peripheral area of the ceramic lid 102 and with the conductivering 124 during the sealing operation.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles in application of theinvention. Thus, it is to be understood that numerous modifications maybe made in the illustrated embodiments and other arrangements may bedevised without departing from the spirit and scope of the invention.For example, the preflowing operation of the present invention may beused to preflow a stamped preform or metallized paste to the peripheralarea of a gold plated Kovar lid to provide an integral heat fusiblelayer defining a hermetic sealing area.

What is claimed is:
 1. A method of fabricating a lid assembly forhermetic sealing of a package containing a semiconductor chip comprisingthe steps of depositing a metallized layer on the surface of said lid atthe periphery thereof, depositing a printed heat fusible solderablelayer over said metallized layer at the periphery of said lid, andpreflowing said integral heat fusible solderable layer thereto.
 2. Amethod of fabricating a lid assembly for hermetic sealing of a packagecontaining a semiconductor chip comprising the steps of depositing ametallized layer on the surface of said lid at the periphery thereof,depositing a heat fusible preform of a solderable alloy over saidmetallized layer at the periphery of said lid and preflowing said heatfusible preform.
 3. The method as set forth in claim 2 further includingplacing a forging weight on top of said lid to enhance the wetting ofsaid metallized layer by said heat fusible preform when said preformbegins to melt.
 4. The method as set forth in claim 2 further includingdepositing an oxidation inhibiting layer between said metallized layerand said heat fusible preform.